1. Field of the Invention
The present invention relates to a method for producing a quartz glass crucible used for the pulling up of silicon single crystals, the quartz glass crucible having a uniform glass layer with fewer internal bubbles.
2. Description of Related Art
Single crystal silicon is mainly produced by a CZ method. This method is a method for producing single crystal silicon by dipping a seed crystal into a molten silicon liquid at a high temperature contained in a quartz crucible, and then slowly pulling up the seed crystal. The method utilizes a quartz glass crucible of high purity for storing the molten silicon liquid.
The quartz glass crucibles used for the pulling up of single crystal silicon are mainly produced according to an arc melting method. This method includes producing a quartz glass crucible by depositing quartz powder, to a certain thickness, on the inner surface of a rotating mold made of carbon to thus form a quartz powder molded product, and heating and melting the quartz powder by means of arc discharge of an electrode installed on the inner upper side of the mold to vitrify the quartz powder (rotating mold method).
In connection to the above-described method, there is known a method of melting the quartz powder by deaerating the quartz powder molded product under reduced pressure through suction from the mold side, in order to eliminate bubbles inside the glass layer (Patent Document 1 and Patent Document 2). In this melting process, deaerating the quartz powder molded product under reduced pressure (this is referred to as vacuum suction) necessitates increasing the degree of vacuum inside the quartz powder molded product, by sealing the inner surface of the quartz powder molded product by melting the inner side uniformly to form a thin glass layer on the surface.
With regard to the above production method for producing a glass crucible by melting a quartz powder molded product, in conventional production methods in which ark melting is initiated at the center of the mold, the entire inner side of the crucible is simultaneously heated, and thus quartz powder can be melted uniformly; however, since the rim edge on the upper part of the quartz powder molded product has a heat retaining property that is lower than that of the bight section or bottom section of the molded product, the melting rate at the rim edge is slow, and a thin glass layer is not satisfactorily formed on the surface of the rim edge. Thus, the degree of vacuum inside the quartz powder molded product is not elevated, and a crucible having fewer internal bubbles in the glass layer cannot be obtained. On the other hand, if the rim edge is melted under an increased calorific value of arc melting so as to supplement insufficient melting of the rim edge, there is a problem in that the bight section or the bottom section is excessively heated and melts, causing the shape of the quartz powder molded product to become susceptible to disintegration.
[Patent Document 1] Japanese Unexamined Patent Application No. 06-191986
[Patent Document 2] Japanese Unexamined Patent Application No. 10-025184
The invention has been made to address the above-described problems in conventional production methods, and according to an aspect of the invention, there is provided a method of producing a quartz glass crucible having a uniform glass layer with fewer internal bubbles.